U.S. patent number 6,397,677 [Application Number 09/587,974] was granted by the patent office on 2002-06-04 for piezoelectric rotational accelerometer.
This patent grant is currently assigned to Kistler Instrument Corporation. Invention is credited to Michael D. Insalaco, Norton Kinsley.
United States Patent |
6,397,677 |
Kinsley , et al. |
June 4, 2002 |
Piezoelectric rotational accelerometer
Abstract
A rotational accelerometer using piezoelectric material,
preferably quartz, in a shear orientation. Piezoplates and
conducting seismic masses, each having bores therethrough, are
bolted to posts that are symmetrically mounted to a body in such a
manner that the bolt passes through the piezoplates but does not
make contact. The accelerometer can be assembled as a single-axis
accelerometer by mounting a pair of posts symmetrically about the
body along the measured axis; additionally, the accelerometer may
be assembled as a double or triple axis accelerometer by
symmetrically mounting additional pairs of posts to the body. The
total weight of the seismic masses and the crystals of the
shear-type accelerometer halves should be equal. The invention sets
forth a novel rotational accelerometer that reduces or eliminates
the need for signal-processing electronics.
Inventors: |
Kinsley; Norton (Lockport,
NY), Insalaco; Michael D. (Niagra Falls, NY) |
Assignee: |
Kistler Instrument Corporation
(Amherst, NY)
|
Family
ID: |
24351945 |
Appl.
No.: |
09/587,974 |
Filed: |
June 6, 2000 |
Current U.S.
Class: |
73/514.34;
310/329; 310/333 |
Current CPC
Class: |
G01P
15/0888 (20130101); G01P 15/0915 (20130101); G01P
15/18 (20130101) |
Current International
Class: |
G01P
15/08 (20060101); G01P 15/09 (20060101); G01P
015/08 (); H01L 041/08 () |
Field of
Search: |
;73/514.34,493,514.36,514.37,862.043,862.638,654,651
;310/329,333 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"The Development of Six-Axis Arrayed Transducer"--Shumin Li, David
L. Brown, Armin Seitz and Mike Lally..
|
Primary Examiner: Kwok; Helen
Attorney, Agent or Firm: Barnes & Thornburg
Claims
We claim:
1. A rotational accelerometer comprising:
first and second spaced-apart linear accelerometers each having
first and second piezoplates and corresponding first and second
seismic mass clamped to a corresponding post; the piezoplates of
each linear accelerometer having parallel first axes of sensitivity
and being electrically connected to be sensitive to linear
acceleration along the parallel axes;
each of the post being connected to a body so that the first axes
of both linear accelerometers are parallel; and
the polarity of the linear accelerometers being opposed and
electrically connected to be sensitive in combination to rotational
acceleration about a second axis perpendicular to the first
axes.
2. The accelerometer of claim 1, wherein the posts of the first and
second linear accelerometers are substantially collinear and extend
substantially orthogonally from the body.
3. The accelerometer of claim 1, wherein
the post, the piezoplates, and the seismic masses of the first
linear accelerometer each have a first bore therein;
the post, the piezoplates, and the seismic masses of the second
linear accelerometer each have a second bore therein;
a first metal bolt extending through each of the first bores and
clamping each of the piezoplates and seismic masses to the post,
the first bolt contacting neither the piezoplates nor the post;
and
a second metal bolt passing through each of the second bores and
clamping each of the piezoplates and seismic masses to the post,
the second bolt contacting neither the piezoplates nor the
post.
4. The accelerometer of claim 3, further comprising:
an electrical connection between the first bolt and a first
electric terminal;
an electrical connection between the second bolt and the first
electric terminal; and
an electrical connection between the body and a second electrical
terminal, such that an electrical instrument can read a signal from
between the first and second electrical terminals.
5. The accelerometer according to claim 3, wherein each bolt has a
smaller diameter than diameter of the bores on the post so as to
create a non-contact annulus therebetween.
6. The accelerometer according to claim 1, wherein each piezoplate
is a shear type quartz piezoplate.
7. The accelerometer according to claim 1, wherein the piezoplates
have equal dimensions.
8. The accelerometer according to claim 1, wherein the posts and
the body are metal.
9. The accelerometer according to claim 1 wherein the body and the
posts are formed as a monolithic, one-piece, metal structure.
10. The accelerometer according to claim 1 wherein the piezoplates
are quartz.
11. The accelerometer according to claim 3 wherein a total of mass
of the seismic masses and piezoplates of each linear accelerometer
is substantially the same.
12. The accelerometer according to claim 1, further comprising a
common housing enclosing the first and second linear
accelerometers.
13. The rotational accelerometer of claim 1, including:
third and fourth spaced-apart linear accelerometers each having
first and second piezoplates and corresponding first and second
seismic mass clamped to a corresponding post; the piezoplates of
each of the third and fourth linear accelerometer having axis of
sensitivity parallel to the first axes and being electrically
connected to be sensitive to linear acceleration along the parallel
first axes;
each of the post being connected to a body so that the first axes
of both of the third and fourth linear accelerometers are parallel;
and
the polarity of the third and fourth linear accelerometers being
opposed and electrically connected to be sensitive in combination
to rotational acceleration about a third axis orthogonal to the
first and second axes.
14. The rotational accelerometer of claim 13, wherein the posts of
the third and fourth linear accelerometers are substantially
collinear and extend substantially orthogonally from the body and
orthogonal to the post of the first and second linear
accelerometers.
15. The rotational accelerometer of claim 13, wherein:
the post, the piezoplates, and the seismic masses of the first
linear accelerometer each have a first bore therein;
the post, the piezoplates, and the seismic masses of the second
linear accelerometer each have a second bore therein;
a first metal bolt extending through each of the first bores and
clamping each of the piezoplates and seismic masses to the post,
the first bolt contacting neither the piezoplates nor the post;
a second metal bolt passing through each of the second bores and
clamping each of the piezoplates and seismic masses to the post,
the second bolt contacting neither the piezoplates nor the
post;
the post, the piezoplates, and the seismic masses of the third
linear accelerometer each have a third bore therein;
the post, the piezoplates, and the seismic masses of a fourth
linear accelerometer each have a fourth bore therein;
a third metal bolt extends through each of the third bores and
clamps each of the piezoplates and seismic masses to the post, the
third metal bolt contacting neither the piezoplates nor the post;
and
a fourth metal bolt passing through each of the fourth bores and
clamping each of the piezoplates and seismic masses to the post,
the fourth bolt contacting neither the piezoplates nor the
post.
16. The accelerometer of claim 15, further comprising:
an electrical connection between the first bolt and a first
electric terminal;
an electrical connection between the second bolt and the first
electric terminal;
an electrical connection between the body and a second electrical
terminal, such that an electrical instrument can read a signal from
between the first and second electrical terminals;
an electrical connection between the third metal bolt and a third
electric terminal;
an electrical connection between either the fourth bolt or one of
the masses of the fourth linear accelerometer and the third
electric terminal; and
an electrical connection to a second electric terminal, the second
electric terminal comprising a ground, enabling an electrical
instrument to read a signal from between the second and third
electric terminals.
17. The accelerometer as in claim 16, wherein the first, second,
third and fourth linear accelerometer's posts are metal and are
electrically connected to one another.
18. The accelerometer of claim 17, wherein the body, as well as the
posts are all formed as a monolithic, one-piece metal
structure.
19. The rotational accelerometer of claim 1, including:
third, fourth, fifth and sixth spaced-apart linear accelerometers
each having first and second piezoplates and corresponding first
and second seismic mass clamped to a corresponding post;
the piezoplates of each of the third and fourth linear
accelerometers having axis of sensitivity parallel to the second
axis and being electrically connected to be sensitive to linear
acceleration along the second parallel axes;
the piezoplates of each of the fifth and sixth linear
accelerometers having axis of sensitivity parallel to the third
axis and being electrically connected to be sensitive to linear
acceleration along the parallel third axes;
each of the post of the third and fourth linear accelerometers
being connected to a body so that the parallel second axes of both
of the third and fourth linear accelerometers are parallel;
each of the post fifth and sixth linear accelerometers being
connected to a body so that the parallel third axes of both of the
fifth and sixth linear accelerometers are parallel;
the polarity of the third and fourth linear accelerometers being
opposed and electrically connected to be sensitive in combination
to rotational acceleration about the third axis; and
the polarity of the fifth and sixth linear accelerometers being
opposed and electrically connected to be sensitive in combination
to rotational acceleration about the first axis.
20. The rotational accelerometer of claim 19, wherein the posts of
the third and fourth linear accelerometers are substantially
collinear; the posts of the fifth and sixth linear accelerometers
are substantially collinear; and the posts of the third and fourth
linear accelerometers, the posts of the fifth and sixth linear
accelerometers and the post of the first and second linear
accelerometers are substantially mutually orthogonal.
21. The rotational accelerometer of claim 19, wherein:
the post, the piezoplates, and the seismic masses of the first
linear accelerometer each have a first bore therein;
the post, the piezoplates, and the seismic masses of the second
linear accelerometer each have a second bore therein;
a first metal bolt extending through each of the first bores and
clamping each of the piezoplates and seismic masses to the post,
the first bolt contacting neither the piezoplates nor the post;
a second metal bolt passing through each of the second bores and
clamping each of the piezoplates and seismic masses to the post,
the second bolt contacting neither the piezoplates nor the
post;
the post, the piezoplates, and the seismic masses of the third
linear accelerometer each have a third bore therein;
the post, the piezoplates, and the seismic masses of the fourth
linear accelerometer each have a fourth bore therein;
a third metal bolt extends through each of the third bores and
clamps the piezoplates and seismic masses to the post, the third
metal bolt contacting neither the piezoplates nor the post;
a fourth metal bolt passing through each of the fourth bores and
clamping the piezoplates and seismic masses to the post, the fourth
bolt contacting neither the piezoplates nor the post;
the post, the piezoplates, and the seismic masses of the fifth
linear accelerometer each have a fifth bore therein;
the post, the piezoplates, and the seismic masses of the sixth
linear accelerometer each have a sixth bore therein; and,
a fifth metal bolt extends through each of the fifth bores and
clamps the piezoplates and seismic masses to the post, the fifth
metal bolt contacting neither the piezoplates nor the post;
a sixth metal bolt passes through each of the fifth bores and
clamps the piezoplates and seismic masses to the post, the sixth
bolt contacting neither the piezoplates nor the post.
22. The accelerometer of claim 21, further comprising:
an electrical connection between the first bolt and a first
electric terminal;
an electrical connection between the second bolt and the first
electric terminal;
an electrical connection between the body and a second electrical
terminal, enabling an electrical instrument to read the signal from
between the first and second electrical terminals;
an electrical connection between the third metal bolt and a third
electric terminal;
an electrical connection between either the fourth bolt or one of
the masses of the fourth linear accelerometer and the third
electric terminal, enabling an electrical instrument to read the
signal from between the second and third electric terminals;
an electrical connection from the fifth bolt to a fourth electrical
terminal;
an electrical connection from the sixth bolt to the fourth
electrical terminal, enabling
an electrical instrument to read the signal from between the fourth
and second electric terminals.
23. The accelerometer as in claim 21, wherein the posts are metal
and electrically interconnected with one another.
24. The accelerometer as in claim 21, wherein the body and each of
the posts are a monolithic, one-piece metal structure.
25. A method of calibrating a rotational accelerometer comprising
the steps of:
selecting a pair of first piezoelectric plates, a pair of first
seismic masses, and one first bolt;
selecting a pair of second piezoelectric plates, a pair of second
seismic mass, and one second bolt;
ensuring that the total weight of the first plates, the first bolt,
and the first seismic masses is equal to the total weight of the
second piezoplates, the second bolt, arid the second seismic
masses;
constructing a first shear-type linear accelerometer from the first
piezoplates and the first seismic masses;
constructing a second shear-type linear accelerometer from the
second piezoplates and the second seismic masses;
aligning the first and second linear accelerometers on a body to
have opposed polarity and parallel axes of sensitivity; and
electrically connecting the linear accelerometers to a common port
so as to measure in combination angular acceleration about an axis
perpendicular to the axes of sensitivity.
26. The method of claim 25, wherein the piezoelectric plates are
selected to be shear plates made of quartz.
Description
BACKGROUND AND SUMMARY OF INVENTION
The present invention relates generally to accelerometers employing
piezoelectric materials, and more specifically to shear
piezoelectric sensors responsive to acceleration or vibration.
The acceleration experienced by a rotating member or structure is
often a very important parameter to consider during a system
design. For example, an automobile crash imparts tremendous energy
to the occupants and significant rotational inertia is also
present. Mechanical structures deform dynamically at resonant
frequencies and the resulting stresses can cause tremendous
damage.
A Finite Element Analysis (FEA) is typically employed to form a
mathematical model of the system. This analysis relates the
deformation at one surface of a discrete elemental section to the
surface deformation at opposing and adjacent elemental surfaces by
an appropriate stress strain relationship. As surface displacements
and rotations are considered in the analysis where each of them
represents a degree of freedom of the system. Attachments such as
welds, joints, bolts or the like can introduce significant error
into the FEA model because the required stiffness estimates are
generated from engineering judgement and empirical data. Therefore,
a dynamic measurement or analysis must be performed when the
results may have critical consequences.
An experimental study such as the above FEA is typically performed
using only linear accelerometers. A spatially narrow array provides
a means to estimate rotations; however, measuring rotations still
presents a great deal of difficulty at interfaces such bolted
joints. These interfaces often have large relative rotation but
vary minimal linear displacement and therefore a method of
measuring rotational acceleration is very important. Unfortunately,
measuring this dynamic rotational data has not been straightforward
due to the lack of convenient and accurate rotational
accelerometers.
A variety of techniques have been attempted which use a pair of
spatially separated, sensitivity-matched linear accelerometers to
estimate and determine rotational acceleration. When linear
accelerometers are located on a fixture at a prescribed distance
apart, the output signal difference between them is used to
estimate rotational acceleration. However, a problematic fact is
the prevailing levels of output signal generated by the
translational movement tends to overshadow those due to rotational
motions. This makes the differencing operations above liable to
serious error. See, D. J. Ewens, Modal Testing: Theory and
Practice; Research Studies Press Limited 1984.
It is the purpose of the present invention to provide a transducer
element or accelerometer which allows the accurate measurement of
angular acceleration which may be suitable for vehicle impact
testing, though not limited solely for this.
Manufacturers of accelerometers have more control over the
sensitivity matching process and can incorporate technologies which
have the qualities required by the design constraints of a
rotational accelerometer. The design of an accelerometer always
involves the optimization of a parameter compromise; thus, there is
not a single accelerometer that fulfills all realms of acceleration
measurement.
Application specific designs are tailored for their best fit into
the field of interest. For example, Experimental Modal Analysis
(EMA) is a field of study which predominately incorporates a sensor
well-suited for low frequency ranges (less than 1000 Hz) and
controlled environmental conditions. A piezoelectric bimorph is
perhaps best for this set of conditions.
A bimorph is formed from two piezoelectric plates which are
inversely polarized, then sandwiched and fused together, then
sliced to form a rectangle. The piezoelectric element of a bimorph
serves as a seismic mass since it is mounted in a manner showing
flexure when exposed to acceleration. When the bimorph is packaged
in a cantilever beam arrangement, the rectangular shape results in
an extremely flexible seismic system, in its sensitive axis, as
compared to the two orthogonal directions defining planes
transverse thereto. Even though the seismic system is not extremely
stiff, as is typical to most accelerometers, the obtainable
frequency response is well-suited for EMA.
The typical bimorph piezoelectric accelerometer is symmetric about
a central fulcrum. Any rotation about this central fulcrum
generates equal magnitude, but inverted, charges from each of the
symmetric beams and therefore a self-cancellation occurs. Central
rotations flex the symmetric halves in opposite directions, while
linear acceleration creates similar bending on both sides of the
fulcrum. When the beams are arranged to have opposite polarity,
however, the charges then sum to provide an output proportional to
angular acceleration about the fulcrum. Such a system is shown, for
example, in U.S. Pat. No. 4,996,878.
The prior art beam-type accelerometer, as disclosed in U.S. Pat.
No. 4,996,878, was capable of detecting rotational acceleration.
The beam-type design required electrical contacts or leads
interconnecting each the stressed faces of the beams. Each of these
leads was connected to a miniature charge amplifier and a miniature
multiconductor cable. The low impedance voltage outputs from each
of the beams are then connected to a remote signal conditioner,
which has facility for powering the sensor's internal electronics
and processing the independent signals from the faces of the beams.
Additionally, the prior art required precision adjustable
potentiometers to precisely fine-tune the sensitivity of the signal
from each face in order to create an exact match to its
counterpart. One summing amplifier and one differencing amplifier
would then provide as output the sums and differences of the sensed
charges. These outputs would be proportional to the linear and
angular acceleration sensed. Thus, the prior art required
post-processing electronics in order to obtain readable figures.
The addition of these post-processing components necessarily
introduces numerous aspects of error into the design.
As alluded to above, the bimorph beam-type accelerometer required
precise sensitivity matching of the beams. This is not a simple
task because even a minor difference in sensitivity can introduce
excessive error. It has been shown that an error in sensitivity
matching as small as one fourth of one percent can contribute 12.3%
error in the computed rotational acceleration even on a simple
cantilever beam structure. See, Shumin Li, David L. Brown, Armin
Sietz, Milte Lally's "The Development of Six-Axis Arrayed
Transducer" Proceedings International Modal Analysis Conf. 1994.
Therefore, it is clear that producing an accurate rotational
accelerometer from commonly available commercial hardware is very
difficult, yet of vital importance in design.
Moreover, bimorph beams necessarily involve fusing two distinct
crystals together with a type of epoxy. The introduction of this
epoxy into the crystalline structure, or even a slight glitch in
the epoxy attaching the beams to the fulcrum, will also introduce
another possibility of error, and may further require frequent
recalibration.
There are several other shortcomings to the beam-type bimorph
design. The beam-type bimorph design is not well-suited for
applications outside of EMA. A preferred design for other higher
frequency or higher-impact applications, such as in crash testing,
is in the shear-type accelerometer. For higher-frequency
applications, accelerometers utilizing the shear principle have
some special advantages over beam-type bimorph accelerometers.
The shear-type rotational accelerometer is assembled in much the
same way as the bimorph beam-type accelerometer. A pair of
accelerometers are aligned with polarities reversed such that their
axes of sensitivity are parallel and spaced-apart. A first axis is
thereby formed to be parallel to, and running between, the axes of
sensitivity. A rigid connection extends between the two
accelerometers, and thereby defines a second axis that runs
perpendicular to each axis of sensitivity, and consequently
perpendicular to the first axis.
A typical model of a shear-type linear accelerometer is described
in U.S. Pat. No. 5,512,794 to Kubler, et al., shown in FIG. 1 as
prior art, known as a K-Shear accelerometer from Kistler Instrument
of Amherst, N.Y. The K-Shear accelerometer is sturdy and
well-suited for high-frequency testing, because the bending
stresses/strains are greatly reduced due to the configuration of
the crystals and seismic masses. Because the potential for error is
greatly reduced, the K-Shear is well-suited for accurately
detecting and measuring acceleration in high-frequency
applications.
In order to give a full understanding of the present invention, the
disclosure of the '794 Kubler et al U.S. Patent is hereby
incorporated by reference.
The post, the piezoplates and the masses each have a
similarly-sized bore passing through. The bores in the post, the
piezoplates, and the masses are aligned so that a bolt can pass
through without making contact with the piezoplates or the post or
z-axis.
The K-Shear accelerometer provides a novel means of detecting
linear acceleration in the direction of the post.
Experience has shown that quartz is a preferred piezoelectric
material because its piezoelectric qualities are substantially
constant, and remain substantially constant over time. Of course,
other piezoelectric materials, including but not limited to
ceramics or even bimorph materials, have been used.
The piezoelectric coefficient of quartz is absolutely relatively
constant and does not change very little, if at all. Accordingly, a
piezoelectric accelerometer was fixed distance between the seismic
masses will not affect the capacitance as originally calibrated,
the accelerometer would produce accurate reading without having to
bear the expense of re-calibration.
Additionally, experimentation has shown that the charge output of a
quartz accelerometer is mass-dependent and does not depend upon the
geometric configuration of the crystal. Thus, assembly of an
accurate shear quartz rotational accelerometer requires one to
carefully determine that the total mass of the crystals and assure
the seismic masses is equal to the combined mass of the counterpart
crystal and seismic mass. The dimensions of the crystals will not
affect accuracy of the accelerometer.
Even though the configuration of the crystals will not affect the
piezoelectric qualities of the crystal, the configuration of the
crystals will affect the signal output to some degree. Varying
thicknesses of crystal between seismic masses will therefore create
varying capacitance between the seismic masses. The configuration
of the crystals, therefore, will affect the capacitance between the
two seismic masses which alters rotational sensitivity but not
accuracy.
As mentioned above, the output signal will be proportional to the
rotational acceleration about the measured axis. The variance in
capacitance, therefore, will be a parameter that will affect the
constant of proportionality between the output signal and the
acceleration. Once this proportionality constant is
determined--hence, the accelerometer is calibrated --then the
shear-type rotational accelerometer will remain remarkably
accurate.
The K-Shear accelerometer has proven to be very reliable, durable
and accurate. To date, however, this design has not been used
independently to determine rotational acceleration. The instant
invention intends to improve upon the current art by providing an
accurate and dependable shear-type rotational accelerometer.
FIRST EMBODIMENT: SINGLE-AXIS ROTATIONAL ACCELEROMETER
The rotational accelerometer has first and second spaced-apart
linear accelerometers halves aligned to have parallel axes of
sensitivity. Each of the first and second linear accelerometer
halves has at least one piezoplate and at least one seismic mass
firmly clamped to the piezoplate.
A rigid connection extends between the linear accelerometer halves
to define a first axis perpendicular to each of the axes of
sensitivity. The polarity of the linear accelerometer halves is
reversed such that each will output a signal of like polarity when
subjected to rotation about a first measured axis that is
perpendicular to the first axis.
The rigid connection comprises a body, a first post extending
substantially orthogonally from the body, a second post extending
from the body substantially collinear to the first post. The first
linear accelerometer half is connected to the first post and
comprises at least one first piezoplate and at least one first
mass, and the second linear accelerometer half is connected to the
second post and comprises at least one second piezoplate and at
least one second mass.
The first post, the first piezoplate, and the first seismic mass
each have a first bore therein, and the second post, the second
piezoplate, and the second seismic mass each have a second bore
therein. A first metal bolt extends through each of the first bores
to clamp each of the first piezoplates and first seismic masses to
the first post in such a way as to ensure that the first bolt
contacts neither the first piezoplates nor the first post.
A second metal bolt passes through the second bore to clamp each of
the second piezoplates and second seismic masses to the second post
in such a way as to ensure that the second connection support
contacts neither the second piezoplates nor the second post.
Preferably, each bolt has a smaller diameter than the diameter
bores on its respective post so as to create a non-contact annulus
therebetween.
There is an electrical connection between the first bolt and a
first electric terminal, and an electrical connection between the
second bolt and the first electric terminal, as well as an
electrical connection between the rigid connection and a second
terminal, such that the second electrical terminal is a ground and
an electrical instrument may read the signal from between the first
and second electric terminals. Impedance buffering electronics are
included in the practical design just prior to the first electrical
terminal for user convenience.
Each piezoplate is a shear type piezoplate, comprising stable
quartz, having nearly equal dimensions. The rigid connection is
metal. The body and the posts may be formed as a monolithic,
one-piece, metal structure. Care should be taken to insure that
seismic mass and piezoplate of each accelerometer half have exactly
the same mass. A common housing encloses the first and second
accelerometer halves.
SECOND EMBODIMENT: DOUBLE AXIS ROTATIONAL ACCELEROMETER
A double-axis rotational accelerometer may be constructed by adding
third and fourth spaced-apart linear accelerometer halves, aligned
so that the first, second, third and fourth accelerometers halves
have mutually parallel axes of sensitivity. Each of the third and
fourth linear accelerometer halves have at least one piezoplate and
at least one seismic mass firmly clamped to the piezoplate.
The rigid connection between the third and fourth linear
accelerometer halves defines a second axis perpendicular to the
first axis. The polarity of the third and fourth linear
accelerometer halves is reversed such that each half will output a
signal of like polarity when subjected to rotation about the second
axis.
The rigid connection has a body connected to the first and second
linear accelerometer halves and a third post extending from the
body and mutually perpendicular to the body and the first post as
well as a fourth post extending from the body substantially
collinear to the third post.
The third linear accelerometer is connected to the third post and
comprises at least one third piezoplate and at least one third
seismic mass, and the fourth linear accelerometer half is connected
to the fourth post and comprises at least one fourth piezoplate and
at least one fourth seismic mass.
The third post, the third piezoplate, and the third seismic mass
each have a third bore therein. Similarly, the fourth post, the
fourth piezoplate, and the fourth seismic mass each have a fourth
bore therein. A third metal bolt extends through each of the third
bores to clamp each of the third piezoplates and third seismic
masses to the third post in such a manner as to ensure that the
third metal bolt contacts neither the third piezoplates nor the
third post. In like manner, a fourth metal bolt passing through to
clamp each of the fourth piezoplates and fourth seismic masses to
the fourth post in such a manner as to ensure that the fourth bolt
contacts neither the fourth piezoplates nor the fourth post.
The double-axis accelerometer has an electrical connection between
the third metal bolt and a third electric terminal and an
electrical connection between either the fourth mass and the third
electric terminal. There is also an electrical connection to a
second electric terminal such that the second electric terminal is
a ground, which enables an electrical instrument to read the signal
from between the second and third electric terminals.
The third and fourth posts are metal and are electrically connected
to one another. In order to accomplish this connection, the body,
as well as the first, second, third and fourth posts can be formed
as a monolithic, one-piece metal structure.
THIRD EMBODIMENT: TRIPLE-AXIS ROTATIONAL ACCELEROMETER
A third pair of linear shear accelerometer halves may be added in
order to create a rotational accelerometer capable of measuring
rotation about a third axis. This embodiment comprises each of the
elements of the double axis rotational accelerometer, and adds
fifth and sixth spaced-apart linear accelerometer halves, aligned
to have parallel axes of sensitivity, wherein each of the fifth and
sixth linear accelerometer halves having at least one piezoplate
and at least one seismic mass firmly clamped to the piezoplate. A
rigid connection between the fifth and sixth linear accelerometer
halves, thereby defining a third axis mutually perpendicular to the
first and second axes. The polarity of the fifth and sixth linear
accelerometer halves is reversed such that each accelerometer half
will output a signal of like polarity when subjected to rotation
about the second axis.
The body is connected to the first, second, third and fourth linear
accelerometer halves, and a fifth post extends from the body
mutually perpendicular to the body and the first and third posts. A
sixth post extends from the body substantially collinear to the
fifth post.
The fifth linear accelerometer half is connected to the fifth post
and comprises at least one fifth piezoplate and at least one fifth
seismic mass; similarly, the sixth linear accelerometer half is
connected to the sixth post and comprises at least one sixth
piezoplate and at least one sixth seismic mass.
The fifth post, the fifth piezoplate, and the fifth seismic mass
each have a fifth bore therein. The sixth post, the sixth
piezoplate, and the sixth seismic mass each have a sixth bore
therein. A fifth metal bolt extends through each of the fifth bores
and clamps each of the fifth piezoplates and fifth seismic masses
to the fifth post such that the fifth metal bolt contacts neither
the fifth piezoplates nor the fifth post. A sixth metal bolt passes
through the sixth bores and clamps each of the sixth piezoplates
and sixth seismic masses to the sixth post in such a way as to
ensure that the sixth bolt contacts neither the sixth piezoplates
nor the sixth post.
The triple-axis embodiment requires an electrical connection from
the fifth bolt to a fourth electrical terminal and an electrical
connection from the sixth bolt to the fourth electrical terminal,
as well as an electrical connection from the fifth post to the
second electric terminal, which is a ground terminal. This
connection enables an electrical instrument to read the signal from
between the fourth and second electric terminals.
Preferably, the fifth and sixth posts are metal and electrically
interconnected with one another. In order to accomplish this
connection, the body and each of the posts are a monolithic,
one-piece metal structure.
The invention further comprises a method of constructing the
rotational accelerometer halves herein disclosed. The method
includes aligning a pair of piezoelectric accelerometer halves to
have polarly opposed yet parallel axes of sensitivity, connecting
at least one pair of piezoelectric accelerometer halves with a
rigid connection, electrically connecting the piezoelectric
accelerometer halves to a common port, and selecting a ground. Once
constructed, the rotational accelerometer is calibrated by
subjecting the accelerometer to a plurality of known rotational
accelerations.
The piezoelectric accelerometer halves are shear-type linear
accelerometer halves, each having at least one shear-type
piezoplate and one seismic mass. It is vitally important to
construct the rotational accelerometer halves so that the total
mass of the plates, bolt and seismic masses of one accelerometer
half equals the total mass of the plates, bolt and the seismic
masses of an opposing accelerometer half.
The rigid connection, which may be formed as a one-piece,
monolithic metal structure, should comprise a body and at least one
pair of collinear posts extending from the body to each of the
respective accelerometer halves.
The invention thus provides high precision and simple design for
accelerometers halves having multiple-axis applications.
Although the present invention has been described and illustrated
in great detail, it is to be clearly understood that the same is by
way of illustration and example only, and is not to be taken by way
of limitation. The spirit and scope of the present invention are to
be limited only by the terms of the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of an accelerometer half of the
prior art.
FIG. 2 is a cross-sectional view of an embodiment of the rotational
accelerometer half that is able to measure and detect rotational
acceleration about a single axis.
FIG. 3 is a cross-sectional view of an embodiment of the rotational
accelerometer half that is able to measure and detect rotational
acceleration about two perpendicular axes.
FIG. 4 is a perspective view of an embodiment of the rotational
accelerometer half that is able to measure and detect rotational
acceleration about three perpendicular axes.
FIG. 5 is an electrical schematic of the embodiment shown in FIG.
2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1, shown as prior art, has one through-bolt 5 that extends
from one of the masses 4, passing through piezoplates 3 and center
post 2 without touching them and is threadably received in the
other mass 4. The annulus space 7 between bore and through-bolt 5
avoids mechanical and electrical contact between through-bolt 5 and
piezoplates 3 and center post 2. This contact separation connects
the two masses 4 into a solid unit exerting only shear forces on
the piezoplates 3. No possible bending moments will occur since the
through-bolt 5 has no securement to pivot about. Thus all forces
parallel to the Z axis remain parallel to the Z axis and are not
converted to moments by the through-bolt 5. All compressional
forces introduced by the masses 4 and through-bolt 5 are only
parallel to the axis of the through-bolt 5 and the bores even the
presence of forces parallel to the Z axis.
FIG. 2 depicts a shear rotational accelerometer half capable of
measuring and detecting rotational acceleration about axis A. The
accelerometer half comprises a pair of spaced-apart linear
accelerometer halves 10, 20. A rigid connection 9 connects the two
accelerometer halves 10, 20. The rigid connection 9 comprises a
body 8 and posts 12 and 22. As shown in FIG. 2, the rigid
connection 9 is a generally rectangular solid shape. Of course,
other suitable configurations for the rigid connection 9 are
possible.
A first post 12 extends substantially orthogonally from the body 8
and has a first bore therein. A second post 22 extends
substantially colinear to the first post 12 from the rigid
connection and has a second bore therein. Additionally, the
rotational accelerometer half has at least one first piezoplate 13,
each first piezoplate 13 comprising a first bore. Additionally, at
least one second piezoplate 23 having a second bore therein is
included. Additionally, the accelerometer half has at least one
first mass 14, each having therein a first bore extending
therethrough, as well as at least one second mass 24 having a
second bore extending therethrough. The accelerometer half further
comprises a first bolt 15 passing through each of the first bores
and thereby clamping the first mass 14 and the first piezoplate 13
to the post 12. It is important to note that the bolt 15 passes
through the piezoplate 13 and post 12 without contacting either. As
such, a gap 17 will result separating the bolt 15 from each of the
piezoplate 13 and post 12.
In like and symmetrical manner, a second bolt 25 passes through
each of the second bores in order to clamp the second mass 24 to
the second piezoplate 23. Analogously, bolt 25 passes through each
of the second bores without making contact with the piezoplates 23
or the post 22, thereby forming a gap 27.
As shown in FIG. 2, the bolts, 15, 25 are configured such that one
end is threaded while the other has a head which abuts one of the
masses. Of course, any alternative design for the bolts which would
achieve the clamping of the masses and piezoplates to the posts
without making contact with the piezoplates or the post would work
equally well.
In order to determine angular rotation, the first piezoplates 13
and second piezoplates 23 are shear type quartz piezoplates mounted
with parallel axes of sensitivity, and polarly opposite to one
another. Viewing FIG. 2, in event the accelerometer half as shown
experiences rotation R about axis A, the first piezoplates 13 will
rotate towards the viewer (out of the plane defined by the page
begin viewed), while plates 23 will be moving away from the viewer
(into the page). Consequently, piezoplates 13 will experience shear
force in the polar opposite direction as the second piezoplates 23.
Thus, in the event they are mounted with like polarity with respect
to an axis perpendicular to axis A, a pure rotation of R about axis
A will always result in near-zero total piezoelectric charge
because the forces will be nearly equal and opposite because the
mass, therefore force, acting on each half is exactly equal
therefore, the equal but opposite charges cancel each other.
However, if the plates 13 and 23 are mounted polarly opposite to
one another with respect to an axis perpendicular to axis A, their
reading, when subject to rotation R about axis A, will sum and
provide a signal proportional to the angular acceleration about
axis A.
The preferred piezoelectric material to comprise the plates is
quartz. Quartz has unchanging piezoelectric characteristics. As
such, the piezoelectric qualities will not change over time which
means that once calibrated, the accelerometer half will retain
accuracy.
First piezoplates 13 should have the same mass as second
piezoplates 23. Not only is this required to symmetry purposes, the
charge output from the quartz is mass-dependent. It has been found
through experimentation, and confirmed by theory, that equal total
masses--that is, the mass of the crystal plus the seismic mass and
bolt will output equal electrical charges notwithstanding any
differences in configuration.
Even though the piezoelectric characteristics of a quartz crystal
are independent of the shape of the crystal, the shape of the
crystal will have an effect on the capacitance. However, because
the plates are clamped to the crystal to remain a constant distance
apart, the capacitance will remain constant. Thus, once the
instrument is calibrated, it will remain accurate potentially
indefinitely because the parameters affecting the output (i.e.,
mass of the crystals, the mass of the seismic masses, mass of
bolts, and the capacitance between surfaces bounding the crystal)
will be constant. A detailed explanation follows.
FIG. 5 presents an electrical schematic of the rotational
accelerometer. Each of the capacitors shown in the diagram
represent sone of the accelerometer halves 10, 20. The voltage V
sensed across the parallel capacitor circuit is directly related to
the respective charge q1 and q2 induced by each of the
piezoelectric linear accelerometer halves 10, 20. Each
accelerometer half will have its capacitance c1 anc c2.
The voltage V is related, in general, to capacitance and charge by
the following equation:
Specifically regarding the schematic set forth in FIG. 5, then, the
following identities apply:
Substituting these identities into the general equation results in
the following relation between voltage, capacitance, and
charge:
V=(q1+q2))/(c1+c2)
Because the capacitance c1 and c2 will remain constant, the voltage
V is directly proportional to the input variables q1 and q2.
In this embodiment, it is preferred that the body 8, first post 12
and second post 22 each be constructed of metal. Additionally,
these three parts maybe formed as a single, monolithic unitary
structure. In doing so, not only is manufacture and assembly
simplified, the body and each of the posts become electrically
connected. As such, a potential experienced at any point on the
second post 22 will be equal to the potential on any point of body
8, and likewise equivalent to the potential on any point of first
post 12.
The masses 14 and 24 are preferably made of metal or any other
electrically conductive material. Additionally, first bolt 15 and
second bolt 25 are likewise made of metal as well. As such, the
charge experienced at any point on either first mass 14 will be
equal to the charge experienced on any point of the first bolt
15.
Analogously, the electrical charge present at any point on either
second mass 242ill be equal to the charge present at any point of
second bolt 25. Ideally, the charge present at first mass 14 should
be equal to the charge at second mass 24.
An electrical lead 52 extends from the first bolt 15 to a first
terminal 55. Analogously, an electrical lead 31 extends from second
bolt 25 to the first terminal 55. As shown, each of the leads 51,
52 may be connected first to a coupler 53 that extends to the first
terminal 55; in the alternative, the leads 51, 52 may directly
connect to the first terminal 55.
As noted before, on each of the bolts there is the same charge as
the masses 14, 24 that it clamps. As such, the leads 51, 52 may
also emanate from masses 14, 24 instead. The embodiment wherein the
electrical leads emanate from the masses 14, 24, however, is not
shown.
An electrical lead 50 extends from the body 8 and connects to a
second terminal 54. The second terminal 54 represents a ground
terminal.
As shown in FIG. 2, a potential difference will exist between
terminals 55 and 56. As shown in FIG. 5, the detected voltage V
will be proportional to the rotational acceleration detected. An
internal impedance converting electronic circuit can be
incorporated between terminal 55 and coupler 53 to transform the
high impedance voltage into an environmentally immune low impedance
voltage prior to the exiting the sensor.
DOUBLE AXIS ACCELEROMETER
Referring to the embodiment depicted in FIG. 3, please note that
this embodiment is similar in many ways to the embodiment in FIG.
2. However, this embodiment allows one to observe, detect and
measure acceleration about two perpendicular axes. Specifically,
this embodiment allows one to detect and determine rotation R about
axis A as well as rotation R'about axis B. This embodiment,
however, will be separated into two distinct substructures.
The first substructure comprises a pair of spaced-apart linear
accelerometer halves 110, 120 and the second comprises a pari of
spaced-apart linear acceleometer halves 30, 40.
For the substructures, the body 98 and the first post 112, second
post 122, third post 32 and fourth post 42 are all made of metal
and are mutually connected. Indeed, one may even perform this
embodiment of FIG. 3 as a monolithic, unitary metal structure. Any
point on body 98 will experience the same electrical potential as
any point on any of the first 112, second 122, third 32 or fourth
42 posts.
For the sake of simplicity, the electrical leads and connections
pertaining to the connection of masses 114, 124 and the posts 112,
122 as well as the leads and terminals as shown in FIG. 2 are also
present in FIG. 3. However, these connections were omitted in order
to make a drawing of FIG. 3 easier to understand and visualize.
Third and fourth post 32 and 42, respectively extend generally
orthogonally from the body 98 mutually perpendicular to the body 98
and first post 112. Each of the third 32 and fourth 42 posts bears
a bore therein for the passage of a through-bolt 35, 45
therethrough. At least one third piezoplate 33 also bears a bore
there through, enabling through 35 to pass through the third post
32 as well as the bore in the third piezoplate 33, and thereby
clamp third masses 34 and third piezoplates 33 to third post
32.
In much the same way, fourth piezoplates 43 and fourth masses 44
are clamped to fourth post 42. It should be noted that fourth post
42 extends from the body 98 in such a way as to be substantially
colinear with third post 32.
Third, piezoplate 33 should bear opposite polarity to the fourth
piezoplate 43. A discussion of the significance of the alignment of
polarity has been set forth above.
Each of the bolts 35, 45 are constructed from metal. Additionally,
the masses 34, 44 are likewise constructed of metal, but may be
made of any electrically-conducting material.
When subjected to rotation R' about axis B, bolt 45 will tend to
move toward the viewer while bolt 35 will tend to move into the
page. As such, piezoplate 43 subjected by mass 44 will yield a
positive charge resulting from the inertial shearing from such a
rotation. Analogously, third piezoplate 33 will typically be
exposed to an equal and opposite shear force. Consequently, it is
again important to mount piezoplates 33 and 43 to be of opposing
polarity.
Third mass 34 is connected via electrical lead 52 to an electrical
coupler 53. In like manner, fourth mass 44 (or fourth bolt 45, as
shown in FIG. 3) is electrically connected via electrical lead 151
to electrical coupler 153. Coupler 153 terminates at a fifth
electrical terminal 155. Additionally, an electrical lead 150
extends from third post 32 to a third electrical terminal 154.
Impedance converting electronics may also be incorporated prior to
terminal 155.
The voltage across terminals 154 and 155 will be proportional to
the angular acceleration about axis B.
TRIPLE AXIS ACCELEROMETER
Referring to the embodiment depicted in FIG. 4, this comprises each
of the elements of the single and double axis rotational
accelerometers 10, 20, 30, 40, 110, and 120 with each of the
rotational axis being mutually perpendicular to the other and lack
of the rotational accelerometers 10, 20, 30, 40, 110, 120
connection via body 108.
The invention thus provides high precision and simple design for
accelerometers halves having multi axis application.
Although the present invention has been described and illustrated
in great detail, it is to be clearly understood that the same is by
way of illustration and example only, and is not to be taken by way
of limitation. The use of the K-Shear linear accelerometer half of
FIG. 1 as the two or four accelerometer halves is mainly uses as an
example. Other pairs of shear type spaced accelerometer halves may
be used. The spirit and scope of the present invention are to be
limited only by the terms of the appended claims.
* * * * *